Status is part of it, but there's a perfectly good statistical explanation too.

There is much higher variance among fat people than among thin people. It's the long tail of the distribution. So plus sizes are much more approximate. It's more likely that the clothes won't fit. This also makes the return on each additional size lower -- there may be a lot of plus-size women generally, but they're spread out enough that there aren't a lot of size 16s specifically.

I don't think that accounts for everything, but it is part of it.

You're already seeing more good plus-size fashion, I think, of necessity. It's coming.